The transfer of energy, such as heat or an electric charge, through

a substance. In heat conduction, energy is transferred from

molecule to molecule by direct contact; the molecules themselves

do not necessarily change position, but simply vibrate more or less

quickly against each other

is the transfer of heat by the movement of a fluid

(liquid or gas) between areas of different

temperature. Warm air is less dense than cold air,

and so convection currents can form in the

presence of a temperature gradient